ting a barometer, had to admit on "clearest
evidence that the bone had caused a rise of from twenty to thirty yards,
so delicately is a balloon equipoised in the air." Here, without pausing
to calculate whether the discharge of an ounce or so would suffice to
cause a large balloon to ascend through ninety feet, it may be pointed
out that the record cannot be trustworthy, from the mere fact that a
free balloon is from moment to moment being subjected to other potent
influences, which necessarily affect its position in space. In daytime
the sun's influence is an all-important factor, and whether shining
brightly or partially hidden by clouds, a slight difference in
obscuration will have a ready and marked effect on the balloon's
altitude. Again, a balloon in transit may pass almost momentarily from a
warmer layer of air to a colder, or vice versa, the plane of demarcation
between the two being very definite and abrupt, and in this case
altitude is at once affected; or, yet again, there are the descending
and ascending currents, met with constantly and unexpectedly, which have
to be reckoned with.

Thus it becomes a fact that a balloon's vertical course is subjected
to constant checks and vicissitudes from a variety of causes, and these
will have to be duly borne in mind when we are confronted with the
often surprising results and readings which are supplied by scientific
observers. With regard to the close proximity, without appreciable
intermingling, of widely differing currents, it should be mentioned that
explorers have found in regions where winds of different directions
pass each other that one air stream appears actually to drag against
the surface of the other, as though admitting no interspace where the
streams might mingle. Indeed, trustworthy observers have stated that
even a hurricane can rage over a tranquil atmosphere with a sharply
defined surface of demarcation between calm and storm. Thus, to quote
the actual words of Charles Darwin, than whom it is impossible to adduce
a more careful witness, we find him recording how on mountain heights
he met with winds turbulent and unconfined, yet holding courses "like
rivers within their beds."

It is in tracing the trend of upper air streams, to whose wayward
courses and ever varying conditions we are now to be introduced,
that much of our most valuable information has come, affecting the
possibility of forecasting British wind and weather. It should need
no insis